1. Field of Invention
The present invention relates generally to a semiconductor wafer processing apparatus. More specifically, the invention relates to a semiconductor support pedestal for providing a controllable, uniform temperature distribution across the diameter of a semiconductor wafer.
2. Background of Prior Art
In semiconductor wafer processing, the surface temperature of the wafer is a critical process parameter. Changes in, and gradients across the wafer surface during wafer processing are detrimental to material deposition, etch rate, feature taper angles, step coverage and the like. It is essential to have control over temperature uniformity during wafer processing to enhance processing and minimize undesirable characteristics.
A number of devices have been used in the art to control wafer temperature during processing. One method feeds a chilled fluid through a wafer support pedestal during wafer processing. The fluid removes heat from the wafer support pedestal thus cooling the wafer. This method of cooling the wafer has two inherent problems. First, the response time required to bring a wafer to a desired temperature is relatively long. As such, rapid dynamic control of the water temperature to compensate for rapid wafer temperature fluctuations is not possible. Consequently, the wafer is not maintained as a constant temperature.
A second disadvantage of this method is the inability to control the temperature uniformity across the surface of the wafer. Heat transfer from the wafer to the wafer support pedestal is generally greatest in the center of the wafer and less towards the edges. Since the fluid temperature is generally uniform inside the wafer support pedestal, the wafer cools more rapidly in the center. This causes a temperature gradient across the wafer surface, becoming more severe with increased diameter wafers, e.g., 300 mm. This temperature gradient is one of the primary causes of feature variation in semiconductor wafer processing.
Another method of controlling wafer temperature that provides rapid dynamic control of the pedestal temperature uses thermo-electric devices embedded in the pedestal surface that supports the wafer (i.e., the support surface). These devices are. oriented in a planar array below the support surface of the pedestal. However, within such an array, temperature gradients form between the individual devices, i.e., each device effectively transfers heat at its location while a lesser amount of heat is transferred at the locations immediately adjacent to and between the devices. Such gradients between a plurality of devices cause substantial temperature variation across the wafer, i.e., hot and cold locations are formed. Consequently, process variations may occur across the wafer in response to the temperature variations.
Recently advances in etching what is considered exotic materials, for example copper and iridium, have aggravated the problems of controlling wafer temperatures and temperature uniformity. The high bias power (up to and exceeding 1000 Watts) applied to electrostatic chucks used in etching some exotic materials have contributed significantly to the heat load upon a wafer, requiring cooling of the pedestal in excess of known means. Additionally, the processing temperatures used in etching some exotic materials require temperatures in the range of 200 to 400 degrees Celsius. Such high processing temperatures require a pedestal that can quickly bring a wafer up to and maintain a predetermined processing temperature.
Therefore, there is a need in the art for an apparatus which can quickly bring and maintain a wafer at a predetermined temperature and provide a uniform temperature across the wafer support surface of a wafer support pedestal.
The disadvantages associated with the prior art are overcome by the present invention of a support pedestal for supporting and thermally controlling a semiconductor substrate, i.e., a wafer. The support pedestal has a substrate support, an isolator, an internal plate and an external plate. The substrate support is an electrostatic chuck, a heater, a ceramic body or the like. The isolator, preferably fabricated from a ceramic or other material having low thermal conductivity, has an internal diameter that defines an internal volume. The isolator is fastened between the substrate support and the external plate. The internal plate is fastened to the isolator. The internal plate and substrate support define a gap vented to atmosphere. Both the internal plate and the external plate have internal passages for circulating a heat transfer fluid.
A second embodiment of the invention comprises a support pedestal having a substrate support, an isolator, an internal plate and an external plate. The substrate support is an electrostatic chuck, a heater, a ceramic body or the like. The isolator has an internal diameter that defines an internal volume. The isolator is fastened between the substrate support and the external plate. The internal plate is fastened to the substrate support. The internal plate has a ring shaped section that defines a volume, separating a first section from a second section. The first section is fastened to the substrate support. Both the second section of the internal plate and the external plate have internal passages for circulating a heat transfer fluid.
A process for thermal control of the wafer is also disclosed. Control of the wafer temperature is facilitated by transferring heat between a substrate support and a fluid circulating within an external plate disposed beneath the substrate support. The external plate is separated from the substrate support by an isolator. Heat is transferred to a fluid circulating within the external plate. Additionally, heat is transferred between the substrate support and the fluid circulating within an internal plate that is disposed beneath the substrate support. Another embodiment of the preferred method includes transferring heat through a ring section in the internal place prior to transferring the heat to the fluid. Other embodiments of the method include sensing the wafer temperature and providing a signal that causes one or more electrodes within the substrate support to generate heat.